Method of making corrosion protected articles



June 1957 A. E. CHESTER ET AL 2,796,361

METHOD OF MAKING CORROSION PROTECTED ARTICLES Filed April 15, 1953 FIG! FIGZ .. 8 %mm m MN N M ER V m N E H N P A E LUS LRO ABJ ATT'YS United States Patent O METHOD OF MAKING CORROSION PROTECTED ARTICLES Allan E. Chester, Highland Park, and Bruno R. Jeremias, Chicago, 111.; and Joseph T. Irwin, Lakewood, Ohio, assignors'to Poor & Company, Chicago, Ill'., a corporation'of Delaware ApplicationApril 15, 1953, Serial No. 349,068

4 Claims. (Cl; 117-71) This invention relates to corrosion protected articles and to a method of protecting certain types of articles against corrosion. The inventionis, especially concerned with the corrosion protection ofgalloys of zinc and copper, more particularly white alloys of. zinc and copper, and still more specifically alloys of zinc andcopper which have been over-plated with chromium.

In-producing metal articles, the surfaces of which contain a thin film, layer or coating of a zincand copper alloy over-plated with chromium, we have found that the resultant article does not have very good resistance to corrosion. It is well known that zinc surfaces tend to turn whiteor grayish white with age, a property sometimes referred to as white corrosion, which is apparently due to the formation of oxides and carbonates of zinc.

Chromium cannot be plated directly over zinc because of poor adherence. We have found, however, that chromium can be plated over a zinc-copper alloy coating containing a sufiicient amount of copper to give the desired adherence but the resultant plated article has a tendency to corrode and form white corrosion products similar to the corrosion products formed withzinc. Eventually, if the corrosion continues over a relatively long period of time, red rust corrosion products are formed 'when the metal article is a ferrous metal.

One object of the invention is to provide a new and improved method for-protecting against corrosion alloys of zinc and copper which have "been over-plated with chromium.

A further object of the invention is to provide new and improved manufacture 'of articles containing a surface coating of an alloy of zinc'and copper over-plated with chromium and sealed .Witha sealing composition which affords good protection against corrosion;

Another 'objectofthe invention is to provide new and improved compositions for-protecting against corrosion alloys of zinc and copper'ovenplated with chromium.

Still a further object ofthe invention'is to provide-new and improved manufacture of articles containing a surface coating of=a. white alloy of zinc. and copper over-coated with .ancoating -of"chromium and sealed with a sealing composition .whi-chlaifords good protection against corrosion and does not discolor, strip, or otherwise-harm the chromium or the zincecopper undercoat. Other objects will appear hereinafter.

In accordance with the invention it has been found-that zinc-copper surfaced articles which have been over-plated with chromium can be-protected against corrosion by treating them with a solution of chromic acid (CrOa) in a strong acid, preferably,containing an acid stable wetting and penetrating agent.

The chromic acid solution is preferably prepared in. the form of a concentrate and then applied to the surface of thearticlein a diluted form. of theconcen'trate. The pH of the composition which is applied to the chromium plated article is preferably within the range of 0.75 to 1.75. Too much acid in the composition will attack the chromium and tendto strip the chromium from the article. Too little acid will. not producethe-desired result;

, 2,796,361 Patented June 18, 1957 While the invention is not limited-to any theory, it is believed that the composition described penetrates the pores, fissures and cracks of the chromium plate and re-, acts with the undercoatingof zinc and copper to seal the pores, fissures and cracks in the chromium, thus making it impossible for the undercoating to be attacked by corrosive agents. 7

The sealing treatment may be applied by spraying,dipping or in any-other suitable manner, but it is preferable to immerse the article to be treated in a solution of the sealing composition for a relatively short period of time, usually around 10 to 60 seconds. Better penetration and better results are obtained if the sealing treatment is carried out at elevated temperatures, say-around l20 F, to 200 'F.

The preferred sealing compositio'nis an aqueous solution of chromic acid and hydrochloric acid containing a Wetting and penetrating agent as an optional ingredient. The type of acid employed, however, is subject to variation and other acids can be used provided they are strong enough to produce a pH within the effective treatment range. Examples of such other acids which are readily availableare sulfuric acid; nitric acid and phosphoric acid.

A further improvement in the corrosion protection is obtained by baking the chromium plated article at a temperature of around 250-300" F., or by putting it in boiling water for a few minutes, before the sealing composition is applied thereto.

Tests which have been made on ferrous metal articles electroplated with analloy of zinc and copper, the-u overpl ated with chromium, and finally treated with a sealing composition as herein described, as compared with the same articles to 'which'no sealing composition has been applied, have demonstrated that the salt spray resistance to white corrosion is improved many fold and in some cases more than ten-fold by. the treatment herein described. The salt spray resistance to the formation of red rust is also improved very substantially,

In the practice of the invention it has also been found that zinc-copper alloy surfaced articles can be protected against corrosion either with or without an over-plating of chromium provided certain precautions are taken with respect to the ingredients of the sealer composition and the concentrations of the ingredients therein. In'this aspect'ofthe invention a sealing composition containing chromicacid, sodium dichromate andnitric acid is employedgwith or'zwithout .a wetting and penetrating agent. For some unexplained reason,.acids such as hydrochloric acid produce a discoloration. on zinc-copperalloys but nitric acid does not.

The nitric acid-containing sealing compositions are especially useful in protecting zinc-copper alloy surfaces which havebeen electroplated with chromium, but where the chromium plating .does not adequately cover the entire surface. It is well known that the throwing power of chromium electroplating baths isrelatively poor and when. the article to lbe plated is contouredand contains angles or recesses there are sometimes areas which are not properly coyered'with chromium in such cases the undercoating of the zinc-collar alloy would be exposed and it is preferableito employ the chromic acid-nitric acid sealing composition. Of course, where there is no chromium film or coating over the-zinc-copper alloy surface thelchromic acid-nitric sealing composition should. be employed in. order to provide corrosion protection without discoloration.

The chromic acid-nitric acid sealing composition can be used satisfactorily at .r00m temperature (20-25 C.). The treatment canbe applied by spraying or dippingor in any other suitable manner but goodresults have been obtained by immersing the article to be,treated,for. a period ofabout 10 seconds to. 2.minutes,, depending upon.

the concentration of the treating composition.

In both types of sealing treatments previously described the article, after being immersed or otherwise treated with the sealing composition, is simply rinsed with water to remove any excess of the sealing composition.

In the accompanying drawing:

Fig. 1 illustrates diagrammatically an article made of iron, steel, or other base material capable of being plated, containing an electrodeposited coating or layer 2 of a zinc-copper alloy over-coated with an electrodeposited layer 3 of chromium and sealed with a sealing layer 4 of a sealing composition of the type herein described;

Fig. 2 illustrates a modification of the invention in which the article produced comprises a base layer 5 of ferrous metal, or other suitable material, containing an electrodeposited layer of a Zinc-copper alloy 6 sealed with a sealing layer 7.

The invention will be further illustrated but is not limited by the following examples in which the quantities are stated in parts by weight unless otherwise indicated.

Example I A concentrate is prepared by mixing togetherthe following ingredients in water:

CrO3 ounces per gallon 25.4 38% HCl cc. per gallon 900 40% solution of octyl alcohol sodium sulfate cc. per gallon 177 a pH of around 1.55, a 2% by volume solution of the concentrate has a pH of around 1.3,' and a 5% by volume solution has a pH of around 0.9.

Example II A concentrate is prepared by mixing together the following ingredients in water:

Ounces per gallon CrOs 30 NazCrzOmZI-IzO 68% HNOa- 21.5 40% aqueous solution of octyl acohol sodium sulfate .85

This concentrate is diluted with water to produce a sealing composition containing 2% to 6% by volume of the foregoing concentrate (2-6 cc. of concentrate per 100 cc. of aqueous solution) and articles having a white zinccopper alloy surface or a white zinc-copper alloy surface over-plated with chromium are protected against corrosion by immersing them in this composition. The optimum results are obtained with a 4% by volume solution of the concentrate at a temperature of to C. for an immersion period of about 1 minute.

In this composition a 2% solution by volume of the concentrate has a pH of about 1.3 and a 4% by volume solution of the concentrate has a pH of about 1.0.

Example III A concentrate is prepared by mixing together 952 pounds of chromic acid (CrOs), 1040 pounds of 20 Baurn hydrochloric acid and Water to make 432 gallons. This-composition can be employed as such as a sealing composition for zinc-copper surfaced articles which have been over-plated with chromium by diluting it with more water to 1 to 5 parts of concentrate by volume to 99 to 95 parts of water by volume and immersing the articles there,- in at a temperature of 70 to 200 F.

4 Example IV The same procedure is followed as that described in Example III except that to the concentrate described as in Example III there is added 20.5 gallons of a composition made by mixing together 25 parts of 20 Bau-m hydrochloric acid, 25 parts water and 50 parts of a composition prepared by dissolving 190 pounds of ammonium sulfate in 45 gallons of water, electrolyzing at a current density of 50 amperes per square foot for 34 hours between a nickel anode and a graphite cathode and filtering. The filtrate which is employed as the addition agent to the concentrate described in Example III apparently contains nickel persulfate which acts as an oxidizing agent. It will be understood, however, that the addition of this ingredient is optional.

Example V A sealing composition is prepared by mixing together 20 cc. of a solution of 400 grams per liter of CrOa in water, 10 cc. of 20 Baum hydrochloric acid, 10 cc. water, and 2 cc. octyl alcohol sodium sulfate. The foregoing composition can be varied by using a solution of chromic acid containing 300 to 500 grams of CrO3 per liter in water instead of 400 grams per liter in water.

The white Zinc-copper alloys referred to in the examples preferably have a copper content of 10% to 20%, the remainder being zinc. Where the alloy is over-plated directly with chromium the copper content should preferably be at least 5%. The copper content of the alloy can also be higher than 20% but is usually not higher than 30% by weight. Although the invention is primarily applicable to the treatment of predominantly zinc alloys of Zinc and copper which have a silvery or whitish appearance and have been over-plated with chromium, it will be understood that the invention can be applied where the chromium is plated over brass generally.

The present invention is not concerned with the manner in which the alloy of zinc and copper is applied to a base material or with the manner in which the chromium coating is applied. Any of the methods heretofore used for applying alloys of zinc and copper to iron, steel, or other suitable base material, may be employed to apply the zinc-copper alloy coating for the purpose of this invention. A suitable method involves the use of an alkaline cyanide electrolyte containing approximately 82% zinc and 18% copper with anodes having substantially the same composition. Alternatively, a zinc anode can be used and the copper can be added to the bath chemically. If desired an external dialysis cell can be used to supply the copper ions or both copper and zinc. Because copper dissolves faster than zinc it is also possible to use mixtures of zinc and copper with split anode circuits so that less current is used in dissolving the copper. In general the proportion of zinc and copper in the bath from which the zinc-copper alloy is electrodeposited preferably varies within the range from about 15 to 30 parts of copper to 85 to 70 parts of zinc. It will be understood that the alloy which is electrodeposited will not necessarily have exactly the same relative zinccopper composition as the electrolyte.

When chromium is electroplated over a zinc-copper alloy plate the latter should preferably be a white zinccopper alloy having a high specular reflectivity. The brightness of the zinc-copper alloy can be enhanced by bufiing and there are several methods for producing bright white zinc-copper alloy coating directly out of an electroplating bath or with a secondary treatment. Examples of such methods are described in copending applications U. S. Serial No. 246,335, filed September 12, 1951, now Patent No. 2,734,026; U. S. Serial No. 260,557, filed December 7, 1951, now Patent No. 2,730,492; and U. S. Serial No. 260,578, filed December 7, 1951, now Patent No. 2,700,646, by oneof the joint applicants herein.

The usual methods of chromiumplating can. be employed in order to apply a chromium layer over the electrodeposited zinc-copper layer.

If a wetting and penetrating agent is used in the sealing composition it should be one which is stable under the highly acid conditions and at the same time not precipitated from the bath or destroyed by the high concentraticn of chromic acid present in the sealing composition. Examples of suitable wetting and penetrating agents are octyl alcohol sodium sulfate, isopropyl naphthalene sodium sulfonate, the dioctyl ester of sodium sulfosuccinic acid, isobutyl naphthalene sodium sulfonate, other alkylated aryl sulfonic acids, soya bean amine oxyalkylated with approximately mols of ethylene oxide, containing an average of carbon atoms in its alkyl chain and having an average molecular weight of 714, and other oxyalkylated amines of long chain acids containing 8 to 60 carbon atoms in which the oxyalkyl groups are derived from ethylene oxide, 1,2-propylene oxide or mixtures thereof. The preferred wetting agents of this last mentioned type contain at least 4 mols of alkylene oxide per mol of primary fatty amine, preferably around 10 to 20 mols of alkylene oxide per mol of primary fatty amine, and are soluble and stable in acids in the concentrations employed in the practice of the invention.

The sealing compositions employed in the practice of the invention preferably contain around 1 gram to 10 grams of chromium (Cr) as chromic acid (CrOs) per liter of solution. Smaller amounts produce a less pronounced effect and larger amounts do not substantially add to the improvement in the corrosion protection obtained and may produce discoloration.

The invention provides new and improved corrosion protected articles and makes it possible to seal a coating of a zinc and copper alloy over-plated with chromium to enhance the corrosion resistance. As a result of the invention it is now possible to provide an undercoating for a chromium plate on ferrous metal articles which is relatively inexpensive. At the same time the resultant article is highly resistant to white corrosion and is resistant to the formation of red rust corrosion over a relatively long period of time.

Where the znic-copper alloy surfaced article is not over-plated with chromium special precautions are necessary in order to prevent discoloration of white zinccopper alloys and the types of sealing compositions which can be employed are limited. Accordingly, this aspect of the invention is described and claimed separately in our application Serial No. 349,105, filed of even date herewith.

The expression zinc-copper alloy surfaced article is employed herein to cover any article having an exterior surface of an alloy of zinc and copper regardless of whether the article itself is made of zinc, a zinc-copper alloy, a ferrous metal, or some other metal capable of being coated with a zinc-copper alloy. The expression zinc-copper alloy plated article is employed herein to describe a zinc-copper alloy coated article in which the zinc-copper alloy coating has been applied by an electroplating process. The expression white alloy of zinc and copper" is employed herein to describe alloys of zinc and copper which have a white or silvery appearance as distinguished from alloys of zinc and copper which are yellow in appearance. The expression overplated with chromium is employed herein to describe an article in which a chromium layer or coating has been applied by electrodeposition over the zinc-copper alloy surfaced article.

The invention is hereby claimed as follows:

1. A process of improving corrosion resistance which comprises treating a zinc-copper alloy surfaced article overplated with chromium with an aqueous solution of chromic acid and hydrochloric acid having a pH within the range of 0.75 to 1.75, said alloy of zinc and copper containing 5% to 30% by Weight copper, the remainder being zinc.

2. A process of improving corrosion resistance which comprises treating a zinc-copper alloy surfaced article overplated with chromium with an aqueous solution of chromic acid and nitric acid having a pH within the range of 0.75 to 1.75, said alloy of zinc and copper containing 5% to 30% by weight copper, the remainder being zinc.

3. A process of improving corrosion resistance which comprises treating a zinc-copper alloy surfaced article overplated with chromium with an acidified aqueous solution of chromic acid having a pH within the range of 0.75 to 1.75, said solution containing 1 to 10 grams of chromium per liter of solution, said alloy of zinc and copper containing 5% to 30% by weight copper, the remainder being zinc.

4. A process of improving corrosion resistance which comprises treating a zinc-copper alloy surfaced article overplated with chromium with an acidified aqueous solution of chromic acid having a pH within the range of 0.75 to 1.75, said alloy of zinc and copper containing 5% to 30% copper, the remainder being zinc.

References Cited in the file of this patent UNITED STATES PATENTS 1,615,585 Humphries Jan. 25, 1927 2,128,550 Ford Aug. 30, 1938 2,181,773 Wernlund Nov. 28, 1939 2,438,013 Tanner Mar. 16, 1948 2,463,039 Kellogg Mar. 1, 1949 2,468,825 Iernstedt May 3, 1949 2,494,909 Spruance Jan. 17, 1950 2,502,476 on Apr. 4, 1950 2,528,717 Batcheller Nov. 7, 1950 2,548,419 Chester Apr. 10, 1951 2,650,902 Higgins Sept. 1, 1953 

1. A PROCESS OF IMPROVING CORROSION RESISTANCE WHICH COMPRISES TREATING A ZINC-COPPER ALLOY SURFACED ARTICLE OVERPLATED WITH CHROMIUM WITH AN AQUEOUS SOLUTION OF CHROMIC ACID AND HYROCHLORIC ACID HAVING A PH WITHIN THE RANGE OF 0.75 TO 1.75, SAID ALLOY OF ZINC AND COPPER CONTAINING 5% TO 30% BY WEIGHT COPPER, THE REMAINDER BEING ZINC. 